A variety of different optical sensors are available for providing an image of objects within the field of view of the sensor. One such system known as a `sweep focus ranger` uses a video camera with a single lens of very short depth of field to produce an image in which only a narrow interval of range in object space is in focus at any given time. By using a computer-controlled servo drive, the lens is positioned (or `swept`) with great accuracy over a series of positions so as to view different range `slices` of an object. A three-dimensional image of the object is then built up from these `slices`. The system detects which parts of the object are in focus by analysing the detected signal for high frequency components which are caused by features, such as edges or textured parts, which change rapidly across the scene. Because of this, the system is not suitable for imaging plain or smooth surfaces, such as a flat painted wall, which have no such features.
This limitation is common to all passive rangefinding techniques. One way to overcome the problem is to actively project a pattern of light onto the target objects which can then be observed by the sensor. If this pattern contains high spatial frequencies, then these features can be used by the sensor to estimate the range of otherwise plain surfaces. A particularly elegant way of projecting such a pattern is described in U.S. Pat. No. 4,629,324 by Robotic Vision Systems Inc.
In this prior art, the sensor detects those parts of the target object that are in focus by analysing the image for features that match the spatial frequencies present in the projected pattern. Various analysis techniques such as convolution and synchronous detection are described. However, such methods are potentially time consuming. In an extension of these ideas a further patent by the same company U.S. Pat. No. 4,640,620 describes a method which aims to overcome this problem by the use of a liquid crystal light valve device to convert the required high spatial frequency components present in the image into an amplitude variation that can be detected directly.
The present invention aims to provide a simpler solution to this problem.